Boron is not a high-profile element. Most people would probably be hard pressed to name an application of boron off the top of their heads. But it's actually fairly common: Borax (as in 20 Mule Team Borax) is hydrated sodium borate. Silly putty, an extremely complex and sophisticated molecular engineering job, uses boron cross-links to give it the critical property of being elastic on short time scales and inelastic on long time scales. This is not easy for a material to do, and boron is the key (see silly putty sample below).
Boron is used to strengthen carbon fiber products such as bicycle frames.
Some very nasty boron compounds have also been used as rocket fuel, because they are able to pack more energy output into a smaller, lighter package than conventional fuels. Unfortunately the compounds in question also kill on contact with bare skin in a matter of seconds. So much for boring boron.

Outrageous price quote.
Our first attempt at a boron sample involved Ed calling up a chemical supplier. They helpfully supplied a price quote of $2030 (two thousand and thirty dollars American) for one square inch of boron foil. We respectfully declined the offer. You might think that boron would be fairly inexpensive, given that it's dirt common and is the major component of things like Borax. But it turns out it's insanely difficult to fuse and work with, so formed shapes of it are quite unreasonably priced. We are preserving this price quote as a reminder of how much money can be saved by using eBay instead of chemical companies.Source:Ed Pegg JrContributor:Ed Pegg JrAcquired:15 July, 2002Price: $0/NothingSize: 1"Purity: 0%

Very Odd Lump.
This is a strange-looking lump of solid pure boron. It has several different kinds of surfaces, as you can see if you click on the picture to see the large version. I would love to know more about the means by which this lump was made.
Reader Graham Cowan offers the following theory:

"In 1985, Callery repurchased reserve pentaborane fuel from the military and reprocessed it into elemental boron. When this became unprofitable ..."

Pentaborane, B5H9, is like all B-H compounds unstable with respect to dissociating into B and H2, so the only processing necessary would have been to heat it in the absence of air and water. (Probably in argon.)

Your nice big photo shows nodular surfaces and fracture ones. I think the nodular surface is where the gaseous BH stuff laid boron down on the hot lump.

Sample from the RGB Set.
The Red Green and Blue company in England sells a very nice element collection in several versions. Max Whitby, the director of the company, very kindly donated a complete set to the periodic table table.

Sample from the Everest Set.
Up until the early 1990's a company in Russia sold a periodic table collection with element samples. At some point their American distributor sold off the remaining stock to a man who is now selling them on eBay. The samples (except gases) weigh about 0.25 grams each, and the whole set comes in a very nice wooden box with a printed periodic table in the lid.

Bottle of lumps.
This is a small bottle's worth of boron lumps similar to the single lump listed above. It represents my element tax extracted on a one kilogram can purchased by Max Whitby for use in the series of museum displays we are building together.

I chose this sample to represent its element in my Photographic Periodic Table Poster. The sample photograph includes text exactly as it appears in the poster, which you are encouraged to buy a copy of.

Boron Nitride ceramic disk.
This ceramic-like disk came from a scrap yard and was claimed to be or contain boron. I don't think it's pure boron, but it could well be a boron nitride ceramic. Unfortunately the analytical instruments available to me don't work on low atomic number elements, so I have no good way of testing what it really is.Source:John WechselbergerContributor:John WechselbergerAcquired:15 April, 2004Price: DonatedSize: 4.5"Purity: <50%

Boron carbide engine sabotage can.
Boron carbide is very hard and can thus be used for grinding. In this case, the idea is to pour some of the dark, oily liquid in this can into the oil supply of an engine in order to cause it to grind itself to a halt. I assume the cylinders either become scored to the point that they can't hold compression, or maybe become jammed by the grit. In any case, this is something you would do only under dire circumstances, for example in war (as this can was intended for), or to an ex-girlfriend or something. Not that I'm recommending that, you could get in big trouble.Source:eBay seller 4slghmr87Contributor:Theodore GrayAcquired:8 December, 2007Text Updated:8 December, 2007Price: $66Size: 3"Purity: <20%

Tourmaline (Dravite variant).
I'm not sure why I have this mineral: I think it may have been a free sample included with some other mineral purchase. Lovely, though of relatively undistinguished chemical composition.Source:Theodore GrayContributor:Theodore GrayAcquired:20 September, 2005Price: DonatedSize: 1"Composition:NaMg3Al6(BO3)3[Si6O18](OH)3(OH)

Lanthanum Boride.
From the source:

This is a very old (relatively) research sample of lanthanum hexaboride. If that chemical name means anything to you, then you likely belong to the 0.01% of the living human population that has been privileged enough to have experience in the realm of electron microscopy, or possibly some other advanced technology involvingthe precise blasting of electrons.

The one and only industrial use for this compound (and the reason for this sample's existence) is as a cathode for emitting electrons into some sort of very expensive piece of equipment. Lanthanum hexaboride happens to have a very small "work function," which is basically a quantitative measure of how much energy it takes for the material to spontaneously spit out an electron. Low work function, more electrons per quanta of energy! It gets much more complicated and less fun (math math math) the more questions one asks, so we'll leave it at that.

So, nowadays, very small (around the size of a grain of sand) perfect single crystals of LaB6 are grown to be used as the most common filament in modern electron microscopes. When I say perfect, I mean that the surfaces of the crystal are not far off from being atomically flat. This is not actually accomplished through the growing process, but rather the post-growth treatment and "polishing." Other cathode materials can be used for various reasons (tungsten metal is still common), but lanthanum hexaboride is widely regarded as the best performing and most reliable. Which neatly segues to this sample: one glance tells us that this is not a single crystal, and it is actually a good-sized chunk. This piece dates from approximately 40 years ago, when lanthanum hexaboride was just discovered as an interesting material. I'm not quite sure which particular property was being investigated with this sample, but all of the compound's electronic properties were measured exhaustively--conductivity, Hall effect, and then that nifty work function. The large white patches and holes in the corners are an artifact of this battery of tests--wires were wrapped around the material through the holes, and an epoxy filled with tiny silver particles was applied to cement these measurement wires in place and guarantee a good electrical connection (so this is actually two element samples in one!). As for the wavy surface texture in the back, I'm not sure- certainly it ended up like that from whatever method was used to fabricate this small block of the material, but I'm not sure what might cause that particular appearance. Bathe in the purple glory of this beautiful and interesting compound that was completely unknown to science when your parents were in school (almost no matter who you are).

Compact flash card hard drive.
This is just crazy. When I first heard about these things my jaw literally dropped (not literally). They are obsolete now, having been hopelessly beaten by solid state flash memory, but in their day they were the highest capacity compact memory cards available, up to 8GB by 2008 (by which time 64GB flash memory cards were available).
And they are mechanical hard disk drives. Let me remind you of the dimensions of a compact flash card (type II): 1.4" x 1.7" x 0.2" (36.4mm x 42.8mm x 5mm). The platter in this drive is about 1" (2.5cm) in diameter. It's just crazy small. There's an electric motor spinning the platter, an electro-magnet that moves the read-write heads back and forth, the whole works, plus of course all the control and interface electronics, packing into no space.
I stand in awe of this device.
The platters are aluminum, the electronics are silicon, the wiring is copper, the magnets are neodymium iron boron, and the magnetic coating is iron and cobalt based.Source: Electronics StoreContributor:Theodore GrayAcquired:28 February, 2009Text Updated:1 March, 2009Price: $100Size: 1.75"Composition:AlSiCuCoFeNdB